Aircraft payload apparatus and method

A method of providing apparatus for use on an aircraft, the method comprising:

• providing a first store and a first payload, the first payload being mounted on or at least partially housed within the first store, the first store being for mounting to an external surface of the aircraft such that the combination of the first store and first payload is wholly outside the aircraft;
• acquiring one or more aerodynamic properties, one or more moments of inertia, and the mass distribution of the combination of the first store and first payload;
• providing a second store and a second payload, the second payload being mounted on or at least partially housed within the second store, the second store being for mounting to an external surface of the aircraft such that the combination of the second store and the second payload is wholly outside the aircraft, the second payload being different to the first payload, the second payload comprising a sensor and/or a projecting means, the sensor being for measuring parameters of one or more entities that are remote from the aircraft, the projecting means being for projecting a mark onto a surface;
• and
• configuring the second store and the second payload such that the one or more aerodynamic properties, one or more moments of inertia, and the mass distribution of the combination of the second store and the second payload are substantially the same as those of the combination of the first store and first payload, thereby providing the apparatus.